Method for measurement of analyte concentrations and a semiconductor laser-pumped, small-cavity fiber lasers for such measurements and other applications

ABSTRACT

A hybrid laser comprising a semiconductor pump laser and small-cavity rare earth fiber laser where laser cavities of both lasers are butt coupled or otherwise optically coupled to form a plurality of laser cavities that produce a plurality of emission wavelengths, one which may be the pump laser emission wavelength at the output of the fiber laser thereby forming a multi-wavelength combined output. The wavelengths in such an output may substantially match distinguishing spectral characteristic features along at least a portion of a characteristic optical spectrum of the analyte under examination providing a method of determining the concentration of an analyte in a specimen undergoing examination.

REFERENCE TO RELATED APPLICATIONS

This application is a continuation-in-part of nonprovisional patentapplication Ser. No. 10/932,163, filed Sep. 1, 2004 and entitled,OPTICAL SPECTROSCOPY APPARATUS AND METHOD FOR MEASUREMENT OF ANALYTECONCENTRATIONS OR OTHER SUCH SPECIES IN A SPECIMEN EMPLOYING ASEMICONDUCTOR LASER-PUMPED, SMALL-CAVITY FIBER LASER, which claimspriority to U.S. provisional application Ser. No. 60/499,489, filed Sep.2, 2003 and entitled, OPTICAL SPECTRAL ANALYSIS TECHNIQUE, and is acontinuation-in-part of patent application Ser. No. 10/411,637, filedApr. 11, 2003 and entitled, FIBER EXTENDED, SEMICONDUCTOR LASER, andalso is a continuation-in-part of patent application Ser. No.10/411,636, filed Apr. 11, 2003 and entitled, CONTROL SYSTEM FOR ASEMICONDUCTOR LASER, all of which applications are incorporated hereinin their entirety by their reference.

BACKGROUND OF THE INVENTION

1. Field of Invention

This invention relates generally to optical spectroscopy and moreparticularly to the deployment of hybrid lasers comprisingsemiconductor-pumped, small- or micro-cavity fiber lasers having a wideoperating spectrum that includes multiple wavelengths in the hybridlaser output that substantially match wavelength spectral characteristicfeatures of analytes supporting the noninvasive measurement of analyteconcentrations in a specimen analyte such as, for example, but notlimited to, glucose, urea, alcohol, cholesterol or bilirubin in humantissue.

2. Description of the Related Art

Optical spectroscopy techniques are becoming increasingly important fora variety of applications for remote and non-invasive sensing. Withinthe medical health field, there is an extensive effort underway todevelop the capability to non-invasively, and in real time, monitor theconcentration of blood constituents, also referred to as analytes, suchas alcohol, cholesterol, glucose, blood oxygen level or other such bloodcomponents. In particular, the ability to monitor the blood glucoseconcentration in a noninvasive manner would provide a tremendous benefitto the millions of people suffering from diabetes, which requirescareful monitoring and control of blood glucose levels. Such anoninvasive approach is painless and does not require penetration of theskin to obtain or draw a blood sample. In order to perform the requiredspectroscopy to extract concentrations of blood constituents, currenttechnology will typically use a broadband radiation source to illuminatethe sample under test, either in-vivo or in-vitro, and couple radiationtransmitted through or backscattered from the sample into an opticalspectroscopy unit in order to extract information about the chemicalcomposition of the illuminated material. Such a system is shown, forexample, in U.S. Pat. No. 6,574,490 and the references cited anddiscussed in that patent. Also mentioned in patent '490 are infraredspectroscopy methods employing infrared radiation either in anabsorption mode or a reflective mode relative to analyte sample underexamination. As a particular example, analytes, such as glucose,cholesterol, alcohol or oxygen content in the blood, will absorbdifferently, relative to absorption coefficient across the infraredspectrum. Since there are many different kinds of identifiable analytesin the sample, their absorption characteristics will overlap and varydifferently from one another across the infrared spectrum and beyondsuch that there is no real reliable way to determine one analyte fromanother without some way of focusing, for example, on distinguishingintensities as wavelength spectral characteristic features along thecharacteristic optical spectrum of the analyte. However, how toaccomplish, in a successful manner, the differential determination ofthese spectrum differences relative to one analyte over another in asample in quick and simple way has eluded many of those skilled in theoptical spectroscopy art, particularly since any system employed wouldhave to successfully evaluate a plurality of intensities for aparticular analyte to successfully achieve a reliably accuratemeasurement and final determination of its concentration on a continuousand speedy basis. Also, conventional infrared absorption spectroscopy ishampered by the intrinsic background absorption of water in the infraredspectrum that has strong absorption at spectral characteristic featuresof similar spectral characteristic features of a blood analyte. Water isnearly 98% of the human body. For example, glucose as an analyte hasabsorption spectrum peaks in the range of about 1,800 nm to 3,400 nm.But water also has a high and varying absorbance in this region and,therefore, can represent a constant and serious interference indetermining the concentration of glucose in an in vivo specimen.Consequently it is very challenging to extract accurate measurements ofthe concentration of a specific blood analyte in the presence ofinterferences both from varying concentrations of other blood analytesand the strong and dominant absorption due to the presence of water. Insummary, the optical spectroscopy art has yet to fully realize a lowcost, noninvasive, highly portable analyte sensor which has the clinicalaccuracy required for widespread adoption.

The above mentioned prior art approaches are also cumbersome resultingin large and relatively inefficient measuring systems. The employment ofa white light source consumes considerable power, much of which is notin the spectral ranges of interest. The spectrometer is inherently alarge device with complex mechanical, or at best electro-optical,mechanisms that are sensitive to ambient conditions and poorly suited toapplications where monitoring is desired in small clinics, homes, or asa small portable or wearable unit.

Thus, there is need for a noninvasive optical spectrometer system thatprovides for a compact and portable unit, such a palm unit, that willilluminate a subject sample or analyte at a signal set of pre-determinedabsorption wavelength features, collect these radiation signals from theresult of scattering, reflection and/or absorption by the analyte to beanalyzed, and provide for the precise determination of the relativestrengths of the various spectral components of the collected signalsrelative to their respective initial signal strengths prior to impingingon the analyte. Therefore, there is further need for an effectiveradiation source that can provide multiple wavelength signals allsensitive to distinguishing wavelength spectral characteristic features,such as intensities of an analyte across a wide absorbance spectrum,which is also sufficiently compact and versatile to be of pocket size oremployed in an in vitro manner or embedded in an in vivo manner. Thereis a further need for such a device to be based on a set of technologiesthat are capable of being readily manufactured at low cost and providingmedically high reliability on a repeated basis of use.

SUMMARY OF THE INVENTION

According to this disclosure, a multi-cavity laser having amulti-wavelength output at one end, comprising a semiconductor pumplaser optically coupled to a short cavity fiber laser having a rareearth doped core, the multi-cavity laser having a first resonator cavityformed by the laser cavities of the semiconductor laser and the fiberlaser via a first mirror at one end of the semiconductor laser cavityand a mirror at the one end and at least one second resonator cavityformed in the fiber laser cavity with, respectively, a second and thirdmirror at each end of the fiber lasing cavity forming the secondresonator cavity, where the first, second and third mirrors are fiberBragg gratings.

An optical spectroscopy apparatus is also disclosed that may be employedto determine the concentration of analyte in a specimen that utilizes asingle radiation source which is hybrid laser comprising a semiconductorpump laser and small-cavity rare earth fiber laser doped with a rareearth species where laser cavities of both lasers are butt coupled orotherwise optically coupled to form a plurality of laser cavities thatproduce a plurality of emission wavelengths, one which may be the pumplaser emission wavelength at the output of the fiber laser, therebyforming a multi-wavelength combined output, where the wavelengthssubstantially match distinguishing spectral characteristic featuresalong at least a portion of a characteristic optical spectrum of theanalyte under examination. In lieu of complex data analysis of thesewavelengths to determine values representing the concentration of theanalyte in an examined specimen, the semiconductor pump laser or lasersare modulated as a plurality of tone frequencies, where at least a firstof the modulation frequencies is below the maximum frequency response ofthe fiber laser so that the first modulation effectively modulates thepump emission wavelength and a first emission wavelength of the fiberlaser in the hybrid laser combined output, and at least a second ofmodulation frequencies is above the maximum frequency response of thefiber laser so that the second modulation effectively modulates the pumpemission wavelength but not the first emission wavelength of the fiberlaser in the hybrid laser combined output. Further, one or moreadditional modulation frequencies may be applied to the pump laser whichare intermediate of the first and second modulation frequencies where itis at least responsive to at least one further emission wavelength ofthe fiber laser and also provided in the hybrid laser combined output.

A further feature of this invention is the employment of a hybrid laserdevice comprising a semiconductor pump laser and a small- ormicro-cavity fiber laser that are butt coupled, tightly coupled, orapproximately coupled where the pump laser is the pumping source for thefiber laser that emits at it output one or more wavelengths asdetermined by the fiber laser cavity mirrors as well as may also emit atthe pumping wavelength where the fiber laser cavity becomes a secondlaser cavity for the semiconductor pump laser by the deployment of anadditional mirror or reflector at the end of fiber laser set to the pumpwavelength. A key benefit of the foregoing laser geometry is that byplacing the rare earth fiber into an extended version of thesemiconductor laser cavity, the fiber laser optical cavity becomes aresonator at the semiconductor pump laser wavelength. This pumpwavelength resonance serves to greatly increase the absorptionefficiency of the pump laser radiation in the fiber laser optical cavitywhile maintaining a short physical path length in order to realize acompact laser. This allows relatively weak fiber absorption, such as inthe case of an erbium doped fiber, to be more effectively andefficiently utilized in this hybrid laser geometry. The semiconductorlaser may be a surface emitter laser or VCSEL laser and may also be anedge emitter laser. The planar fabrication process for the VCSEL pumplaser readily lends itself to cost effective hybrid integration ofarrays of such pump lasers, whether linear laser arrays ortwo-dimensional laser arrays, coupled with corresponding rare dopedfiber lasers each having a doped fiber core with one or more rare earthmaterials, such as, for example, thulium, erbium, holmium, ytterbium,neodymium, promethium, terbium, praseodymium, or the like, to producecompact and efficient multi-wavelength radiation source modules foroptical spectroscopy or other applications, for example, but not limitedto, LIDAR. By appropriate design and electronic control of thesemodules, dramatic reductions in size and cost can be achieved forsystems requiring precision wavelength, low noise optical spectroscopy.

It should be further noted, as alluded to just above, in arrays of suchhybrid semiconductor pump lasers/fiber lasers, the cores of each of thefiber lasers in an array can contain all the same rare earth dopant, ordifferent rare earth dopants or combinations of a plurality of differentrare earth dopants. Thus, the novel semiconductor laser/fiber laserarray of this invention enables compact, reliable radiation sourceswhich span a broad combined range of fiber laser wavelengths andsemiconductor laser wavelengths and able to accommodate compactmulti-wavelength sources comprising 4 to 20 or more emission wavelengthsfrom their hybrid laser outputs.

These multi-wavelength radiation sources can be targeted for emissionwithin specific wavelength bands and at specific absorption peaks orother types of optical characteristic features, such as peaks inspectral residuals, that yield optimum spectral information on atargeted analyte or other organic or biotissue as well as targeted forpower levels and modulation characteristics that enable the achievementof high signal to noise (SNR) spectral analysis. As an example of suchtargeted analyte or other organic or biotissue, specific examples arereference to the content of analytes in the blood, such as, glucose,oxygen content, alcohol, ethanol, albumin, urea, lactose andcholesterol.

A further feature of this invention is a noninvasive spectroscopic bloodassay system for the quantitative assessment and measurement of humanblood species, exemplified in the paragraph above, based upon aninnovative laser technology platform that provides for a compact,portable and/or wearable ultra-high resolution, near-infrared,multi-wavelength spectroscopy system. This platform results in a majoradvanced noninvasive monitoring capability for patients requiringcontinuous monitoring of one or more blood analytes. As an example, inthe case of glucose, diabetic sufferers are able to achieve continual,real-time monitoring of blood glucose levels in order to optimizedisease management. Further, the platform is capable of cost effectiveproduction in large quantity in order to provide a reasonably pricedunit for diabetic patients as end users of such units. For example, thesize of such an unit, including its control electronics, is about thesize of a cellular telephone with the capability of providing either ondemand noninvasive sensing and monitoring or continuous noninvasivesensing and monitoring of the blood concentration of specific species ora specific set of blood species, such as, for example, glucose,cholesterol and alcohol, where the latter can have a significant affecton the maintenance of proper glucose and cholesterol levels in the bodywhen the blood alcohol content is too high.

Another feature of this invention relates to optical spectroscopyapparatus for determining the concentration of an analyte in a specimenemploying a hybrid laser that includes a semiconductor pump laser with asmall-cavity fiber laser having a plurality of laser cavities thatprovide at an output from the hybrid laser a plurality of differentemission wavelengths of radiation at least one from the small-cavityfiber laser and another from the semiconductor pump laser thatsubstantially overlap distinguishing wavelength spectral characteristicfeatures along at least a portion of a characteristic optical spectrumof the analyte; means for modulating the hybrid laser with a pluralityof tone frequencies where all the tone frequencies effectively modulatean emission wavelength from the semiconductor laser and at least onetone frequency effectively modulates an emission wavelength from thefiber laser; means for collecting the modulated radiation from thehybrid laser output reflected from or passed through the specimencontaining the analyte under examination; means for sensing themodulated tone frequencies from the collected radiation wavelengthsproducing a plurality of tone frequencies representative of values atthe wavelength spectral characteristic features of the analyte withinthe characteristic optical spectrum portion; means for comparing thesensed tone frequencies with a set of corresponding tone frequenciesabsent of the spectral characteristic features from the analyteproducing a set of values representative of spectral characteristicfeatures after engagement with the analyte; and means for correlatingdifferences in the set of values to produce a final value representativeof a measurement of concentration of the analyte in the specimen.

It is within the scope of this invention that the claimed opticalspectroscopy apparatus may also be employed as an in vivo means todetermine, on a continuous basis, the concentration of a targetedanalyte in the human body.

Other objects and attainments together with a fuller understanding ofthe invention will become apparent and appreciated by referring to thefollowing description and claims taken in conjunction with theaccompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

In the drawings wherein like reference symbols refer to like parts:

FIG. 1 is a cross-sectional side elevation view of a VCSEL or surfaceemitter pumped micro fiber laser adapted for deployment in thisinvention.

FIG. 2 is a cross-sectional side elevation view of a side emitter pumpedmicro fiber laser adapted for deployment in this invention.

FIG. 3 is similar to FIG. 1 except two or N multiple semiconductor/microfiber lasers or laser arrays are illustrated, each of which may beoperating at a different set of wavelengths.

FIG. 4 is a cross-section view similar to FIG. 1 and where micro fiberlaser is modulated with multiple frequency signals comprising twodifferent frequencies.

FIG. 5 is a cross-section view similar to FIG. 1 and where micro fiberlaser is modulated with multiple frequency signals comprising threedifferent frequencies.

FIG. 7 is a graphic illustration of several different types of fibermaterials and their rare earth dopants showing their fiber laseremission wavelength spectral ranges when pumped near 800 nm.

FIG. 6 is a graphical illustration of the absorption spectra over a widerange of wavelengths (200 nm to 1,000 nm) for oxygenated anddeoxygenated hemoglobin.

FIG. 7 is graphic view of a plurality of different rare earths doped incores of different types of glass optical fibers.

FIG. 8 is graphical illustration of the absorption coefficient curve forwater across a wide spectral range (800 nm to 4,000 nm) together withradiation depth penetration, and peak positions of spectralcharacteristic features comprising the spectral characteristic featuresof several biologically interesting analytes or species superimposed onthe water absorption curve.

FIG. 9 is a graphical illustration of glucose absorbance and itsspectral residuals as determined from two different analyticalapproaches, partial least squares spectral analysis and net-analytesignal analysis.

FIG. 10 is a simple schematic diagram of a first multi-wavelengthspectrometer system employing this invention.

FIG. 11 is a simple schematic diagram of a second multi-wavelengthspectrometer system employing this invention.

FIG. 12 is a schematic view of a first embodiment for subsystems thatmay be deployed in the systems of FIG. 10.

FIG. 13 is a schematic view of a second embodiment for subsystems thatmay be deployed in the systems of FIG. 10.

FIG. 14 is an energy-level transition diagram for thulium.

FIG. 15 is an energy-level transition diagram for erbium.

FIG. 16 is another embodiment of cross-sectional side elevation view ofa side emitter pumped micro-fiber laser according to FIG. 2 havingend-cavity fiber gratings forming a fiber laser cavity for deployment inthis invention.

FIG. 17 is another embodiment of cross-sectional side elevation view ofa side emitter pumped micro fiber laser according to FIG. 2 havingcavity fiber gratings, similar to a distributed feedback laser (DFB)cavity, forming a fiber laser cavity for deployment in this invention.

FIG. 18 is a further detailed energy-level transition diagram forthulium.

FIG. 19 is a further embodiment of cross-sectional side elevation viewof a side emitter pumped micro fiber laser according to FIG. 2 havingmultiple end-cavity fiber gratings forming a plurality fiber lasercavities for deployment in this invention.

FIG. 20 is a still further detailed energy-level transition diagram forthulium illustrating multiple cascade transitions where efficient photonextraction can occur.

FIG. 21 is an expanded energy-level transition diagram for thuliumrelative to the second cascade transition.

FIG. 22 is a further embodiment of cross-sectional side elevation viewof a side emitter pumped micro fiber laser according to FIG. 19 havingfurther multiple end-cavity fiber gratings forming a plurality of fiberlaser cavities (three fiber laser cavities) for deployment in thisinvention.

DETAILED DESCRIPTION OF THE INVENTION

As used herein, “small cavity” fiber laser means a comparatively verysmall laser cavity compared to fiber lengths generally employed forfiber lasers in the art. Typical fiber lengths for the VCSEL-pumpedsmall cavity fiber laser employed in this invention may lie in therange, for example, from about 0.5 cm to about 10 cm, although the fiberlength may also lie outside this range. The concentration for the dopantspecies may be in the range from 1,000 ppm to 30,000 ppm and typicalinner core diameters of the fibers may be in the range from about 3 μmto about 10 μm, although it will be appreciated that the dopantconcentration and the fiber core diameter may also lie outside theseranges. Also, as used herein, “spectral characteristic feature” means aminimum, zero or maximum of wavelength variation of an opticalcharacteristic along at least a portion of the characteristic opticalspectrum of an analyte under examination. An optical characteristicfeature may include absorption, transmission or scatteringindependently, or a difference in any of these characteristics relativeto any given set of reference or interfering optical characteristics.Examples of such sets are net analyte signal (NAS) and spectralresiduals.

Reference is now made to FIG. 1 which is also similarly disclosed inU.S. application Ser. No. 10/411,637, supra, as FIG. 6. A firstembodiment of a semiconductor laser-pumped small-cavity fiber hybridlaser 10 is schematically illustrated in FIG. 1 comprising a surfaceemitting semiconductor laser 12 and a rare earth doped fiber laser 14.In the illustrated embodiment of FIG. 1, the semiconductor surfaceemitting laser is also known as a VCSEL, but it will be appreciated bythose skilled in the art that an edge emitting semiconductor laser, suchas illustrated in FIG. 2, to be discussed next, may also be employed asa semiconductor pumping laser.

Thus, hybrid laser 10 includes VCSEL 12 and a micro-cavity orsmall-cavity fiber laser 14. VCSEL 12 comprises substrate 16 on which isformed a vertical optical cavity structure having an optional VCSELoutput coupler and cavity mirror 18, gain region 20, and cavity mirror22, together forming the laser cavity, L1, of pump laser atsemiconductor laser 12. Cavity mirror 22 may be a Bragg high reflectorand VCSEL output coupler 18 may be a Bragg partial reflector, withreflector semiconductor layers of either cavity mirror epitaxially grownover the substrate 16 as well known in the art. Gain region 20 maycomprise an epitaxially grown semiconductor gain layer, and may be, forexample, a single bulk active layer or quantum well/barrier region asknown in the art. The types of semiconductor materials used in the gainregion are typically selected so that the VCSEL 12 oscillates at adesired wavelength. This wavelength is then employed for pumping thefiber laser 14. The types of materials used in the reflectors 18 and 22are typically selected to provide desired amounts of reflectivity at thedesired wavelength, with little or no optical absorption. Reflector 22is typically made from a semiconductor material, such as AlGaAs, thathas high thermal conductivity so as to efficiently conduct heat awayfrom active region 20 to heatsink 28. An electrode 24 may be formed overthe Bragg high reflector 22 for passing electrical current through thegain region 20 to a corresponding electrode on the other side of activeregion as seen at 17. An insulating current barrier 26 may be providedfor lateral confinement of the current passing through the gain region20. Heatsink 28 may be in thermal contact with the epitaxial top ofvertical cavity pump laser 12 to provide for efficient heat dissipation.Substrate 16, which may be GaAs or InP, is provided with an aperture 32to provide an optical output port from VCSEL 12 which output isoptically coupled into rare earth doped fiber 14. A first end 34A offiber 34 receives the coupled radiation from gain region 20 and theoptical cavity of laser 12. Fiber 34 may be attached to substrate 16forming a butt coupling with optional mirror or reflector 18, or may beinserted, at least part way, into aperture 32 and held in coupledposition by means of an optical gel which has a refractive index thatrenders it transparent to pump radiation emitted from laser 12. Bothfiber laser 14 and VCSEL 12 may be supported on a substrate or bench 49.A first fiber reflector 36 is disposed at first end 34A of fiber 34 anda second fiber reflector 38 is disposed at a second end 34B of fiber 34.Fiber reflectors 36 and 38 form a fiber laser cavity for fiber 34.Typically, first fiber reflector 36 is a high reflector at the emissionwavelength of small-cavity fiber laser 14. First fiber reflector 36 mayhave a low reflectivity at the pump emission wavelength so that a largefraction of the pump radiation from VCSEL 12 passes through first fiberreflector 36 into fiber 34. First fiber reflector 36 may be a multiplelayer dielectric mirror deposited on end 34A of fiber 34, or may be areflector structure separate from fiber 34. In one embodiment (notshown) for reflector 36 to be separate from fiber 34, first fiberreflector 36 is epitaxially grown as a Bragg reflector on substrate 16,between gain region 20 and fiber 34. In either case, as mentionedpreviously, first fiber reflector 36 is generally transparent to thepump laser emission wavelength but is highly reflective, i.e., close to100%, of the fiber laser emission wavelength provided as part of thehybrid laser output at 39. Second fiber reflector 38 at fiber end 34Btypically has a reflectivity less than 100% and operates as the outputcoupler for fiber laser 14. The value of the reflectivity of secondfiber reflector 38 may be selected for optimum output coupling, based ona number of system parameters such as the fiber length, pump power,doping level, core diameter, and the like. Also, which is particularlynovel in laser 10, is a pump radiation reflector 40 that may be disposedat second end 34B of fiber 34 for reflecting pump radiation that haspassed through and into the optical cavity L3 of fiber 34 and itsassociated cavity reflectors 36 and 38 are not generally absorbing ofthis pump radiation. This may lead to increased pump absorptionefficiency which permits the use of even shorter fibers for fiber laser14. Furthermore, the device may be configured such that the fiber cavitycoupling to the VCSEL laser cavity is high, and pump radiation reflector40 represents of significant reflectivity contribution to thesemiconductor pump laser cavity. Under this circumstance, one is able toachieve resonantly enhanced absorption of the pump laser radiation inthe fiber cavity.

In the optimum case, the pump radiation absorption in the fiber cavitycan be configured to be the predominant source of photon loss out of thepump laser cavity so that substantially all of the radiation generatedin the semiconductor pump laser cavity is delivered to fiber laser 34.In this case, highly efficient pumping of fiber laser 14 can be realizedwhich, in turn, permits significant reduction in the fiber length downinto the mm range resulting in an optical fiber cavity that is of verysmall or micro size in terms of present day utilized fiber lasers. Pumpradiation reflector 40 and second fiber reflector 38 may each bemultiple layer dielectric mirrors deposited on second end 34B of fiber34, or may be separate formed from fiber 34.

Fiber 34 has a core 42 which is doped with the excitable species, forexample, rare earth doping species such as thulium, erbium, holmium,ytterbium, neodymium, promethium, terbium, praseodymium, or the like. Asa specific example, fiber 34 may have an outside diameter ofapproximately 150 μm and a core diameter of about 6 μm. One or more ofthese dopants are incorporated into core 42 of fiber 34 and are excitedby the pumping radiation from VCSEL 12 which is at the proper pumpingwavelength to carry out such excitation. Emitted radiation at output 39of laser 10 is at a wavelength determined by the specific rare earthspecie or species incorporated into core 42 and the reflectiveproperties of fiber cavity mirrors 36 and 38. For example, if the rareearth dopant chosen for core 42 is thulium, the output 39 of fiber laser14 may yield emission wavelengths in the 2,300 nm range of wavelengthswith suitably designed mirrors. In this connection, see FIG. 14 whichshows the possible emission wavelengths for a thulium doped fiber laser14. As further indicated in FIG. 1, Fiber 34 may comprise a double-cladfiber comprising a first cladding 44 surrounding the rare earth speciesdoped core 42 and a second cladding 46 surrounding first cladding 44.Second cladding 46 is then covered with a protective jacket 47. Fiberlaser radiation is confined to doped core 42 because the effectiverefractive index for the fiber laser radiation is less in first cladding44 than the effective refractive index for doped core 42. Pump radiationfrom VCSEL 12 is coupled into, and is confined by, first cladding 44,since the effective refractive index for pump radiation in firstcladding 44 is higher than the effective refractive index in secondcladding 46. The pump radiation, therefore, passes along, through andcrisscrossing fiber core 42 and first cladding 44. Optical confinementof the pump radiation from pump laser 12 is typically multimodeconfinement. One advantage to employing a double-clad fiber in anembodiment for laser 14 is that the coupling efficiency of pumpradiation propagating from VCSEL 12 into fiber 34 of fiber laser 14 isvery high.

Thus, it can be seen that hybrid laser 10 of FIG. 1 provides for laseremission at two, rather than one wavelength. The VCSEL pump source 12will, at some level, inevitably provide radiation emission at the outputend of fiber laser 14 where pump wavelength reflector 40 is below being100% reflective. Thus, this pump wavelength emission therefore is of anintensity determined by the reflectivity of the VCSEL mirror, M3, at 40.Thus, by controlling the transmissivity magnitude of mirror, M3, theamount of VCSEL pump radiation can be controlled that is allowed to leakthrough mirror 40 as part of the laser output 39. This background pumpradiation may be put to use as another wavelength for monitoring aspectral characteristic feature, for example, such as an absorptionfeature, which may be a peak analyte absorption intensity that is at theVCSEL emission wavelength, together with the fiber emission wavelength,thus creating a two wavelength integrated emission source 10.

It will be appreciated, of course, that fiber 34 need not be double cladfiber, and that the pump radiation from VCSEL 12 may be coupled directlyinto fiber core 44. In such a case, the pump radiation intensity in thecore is higher than that with a double clad fiber, but the overallefficiency for coupling pump radiation into fiber 34 may be reduced.

It will be further appreciated that fiber 34 may be a multimode gradientindex fiber with an imbedded smaller diameter doped core with singlemode or near single mode propagation. This configuration enables thecombination of the previously mentioned coupling and propagationbenefits of the graded index fiber with the wavelength conversionproperties of the excitable species doping. In some embodiments, thelength of fiber 34 may be around 1 cm. For such a length, it may bedesirable to have a relatively high level of doping of excitable speciesin fiber core 42. It may also be desirable to have a relatively largecore diameter. Also, the refractive index profile of fiber 34 need notbe parabolic, and may have some other type of profile, such as acombination of parabolic and refractive index stepped profile or purelyrefractive index stepped profile. More is said about this matter in U.S.patent application Ser. No. 10/411,637, incorporated herein.

In many configurations, the VCSEL lasing cavity is formed between Bragghigh reflector 22 and VCSEL output coupler 18. Pump radiation emitted bythis cavity passes into the fiber cavity formed between the fiberreflectors 36 and 38. The pump radiation is absorbed in doped fiber 34so as to excite the excitable species in fiber 34. Pump radiationreflector 40 at the second end 34B of fiber 44 reflects unabsorbed pumpradiation back through the fiber cavity towards VCSEL 12. Thus, inessence, a pump cavity is also formed between VCSEL output coupler orreflector 18 and pump radiation reflector 40. Where the frequency of thepump radiation falls at one of the resonant frequencies of the pumpcavity, the pump radiation may be circulated many times within the pumpcavity, thus encouraging higher absorption efficiency of the pumpradiation within the fiber 34. It is preferred that the excitablespecies not manifest optical gain at the wavelength of the pumpradiation. Where the pump radiation is resonant in the pump cavity, thelength of fiber 34 employed for fiber laser 14 may be selected more onoptimum gain length, rather than on a minimum length driven by thesingle pass absorption length of pump radiation in fiber 34, as is thetypical approach taken by those skilled in the finer laser design art.

It is also within the scope of this invention to provide a VCSEL 12 thatis tunable. To this end, the gain cavity of the VCSEL may be varied tochange and tune the wavelength output of the VCSEL by means, forexample, of a micro-electrically mechanical mechanism as seen inPublication No. WO02/0844826 or U.S. patent application, Pub. No.2003/0031221A1, respectively, by providing a movable mirror assembly forone of the VCSEL DBR cavity mirrors through an applied voltage or byconstructing one of the DBR cavity mirror assemblies to be comprised ofa MEMs type of structure. Another approach to vary the cavity length ofthe VCSEL is by changing the refractive index of one of the DBR cavitymirrors of the gain cavity of the VCSEL as disclosed in U.S. patentapplication, Pub. No. 2004/0028092. These three patent applicationpublications are incorporated herein by their reference.

Reference is now made to FIG. 2 which illustrates a hybrid laser 50comprising a semiconductor laser pumped small- or micro-cavity fiberlaser which includes small-cavity fiber laser 14 as seen in FIG. 1 but,in the case here, butt coupled to semiconductor side emitter laser 52which is here shown in this embodiment as a Fabry-Perot laser. Laser 52may also be a distributed Bragg reflector (DBR) or a distributedfeedback (DFB) laser with a feedback grating along and adjacent thelaser cavity core. Laser 52 comprises an optical cavity 54 and a pumpstripe 57 with a p-contact electrode 56 and a n-contact electrode 59 asknown in the art. The back and front cavity mirrors 58 and 60 may bedeposited on the laser facets comprising a plurality of dielectriclayers that provide the necessary feedback to cause laser 52 to lase ata desired operational wavelength. However, Fabry-Perot laser 52 may havecavity end facets that provide laser cavity feedback to provide lasingwith the laser cavity being of a designed length to provide an emissionwavelength design capable of pumping micro-cavity fiber laser 14. Thedescription of fiber laser 14 is the same here as discussed inconnection with FIG. 1. It should be noted that there will be somecompromise in the deployment of edge emitter pump lasers 52 rather thansurface emitter lasers 12 in coupling efficiency due to the ellipticalemission beam from these types of edge emitter lasers. However, higheroutput powers are available from edge emitter lasers compared to surfaceemitter lasers. The edge emitter laser also does not readily lend itselfto being fabricated in two dimensional arrays and are much more amenableto linear arrays or bars. Either device, however, can be supported bysimilar control electronics some of which is depicted in FIGS. 10-13, tobe discussed later.

VCSEL pump laser may be used to pump the excitable species into anexcited state through the absorption of one or more pump photons. Onespecific example of a VCSEL pumped laser is a thulium-dopedfluorozirconate (ZBLAN) optical fiber. A portion of the transitionenergy diagram for this system, with a specific absorption/emissionpathway highlighted, is shown in FIG. 14. The pump photon absorbed bythe thulium ion, Tm³⁺, has a wavelength of around 790 nm. Pumping around790 nm is well suited to populating transition states leading to lasertransitions at 2,300 nm, 1,820 nm and 1,470 nm. Potential forsimultaneous lasing at both 1,820 nm and either 2,300 nm or 1,470 nmresults from both 2,300 nm and 1,479 nm laser transitions leading to theexcited state at 3F₄ transition for 1,820 nm. As shown in FIG. 14,absorption of this pump photon raises the thulium ion to the 3H₄transition level. There are at least two decays paths from this level tothe excited state 3F₄ level. One path, which is accompanied by anemission at 2300 nm, which is a two-step decay via the 3H₅ level, andthe other is a direct transition that is accompanied by the emission ofa photon at 1,470 nm. The 3F₄ level has a transition back to the 3H₆ground state level that is accompanied by emission of a photon at 1,820nm. Laser oscillation may be achieved on both the 3H₄-3H₅ and the3F₄-3H₆ transitions so that fiber laser 14 may produce radiation at2,300 nm and/or 1,820 nm. Fiber laser 14 may, thus, be employed toaccess emission wavelengths substantially longer than the emissionwavelengths available currently with VCSEL devices. Such a fiber laseris based on the use of a pump photon, for example, at 780 nm or 790 nm.A VCSEL laser 12 having an AlGaAs active region may be used to generatepump radiation within this wavelength range.

In connection with the transition diagram for thulium of FIG. 14, thetransition ranges of wavelengths, such as 1,460 nm to 1,510 nm, 1,700 nmto 2,015 nm and 2,250 nm to 2,400 nm all include wavelengths having highabsorption sensitivity for the analyte, glucose, which can be utilizedto provide useful information for determining the concentration ofglucose in a specimen. Wavelengths in these ranges, particularly thoserepresenting wavelength spectral characteristic features, such asrepresented by positive or negative peaks for glucose absorption, forexample, can be generated as emission wavelengths from micro-cavityfiber laser 14 by determining and designing the reflectivity spectra formirrors or reflectors 36 and 38 applied to fiber laser 14 to create theoptical cavity, which can be determined with a significant degree offlexibility employing standard thin film optical coating designs anddeposition techniques. Thus, the hybrid laser platform illustrated inFIGS. 1 and 2 enables the realization of a single unitary, compactradiation source with the capability of being able to produce one ormore wavelengths over a broad spectral range of wavelengths, some ofwhich are out of the emission wavelength range of semiconductor lasers,employing a fixed pump wavelength from a semiconductor pump laser, suchas at a pump wavelength of 785 nm, which pump wavelength may also be oneof the emission wavelengths of from hybrid laser 10 or 50.

Reference is now made FIG. 3, and to the semiconductor laser pumpedsmall- or micro-cavity fiber laser array 60 which is another embodimentof the laser that may be utilized in this invention. Laser array 60comprises a plurality of semiconductor laser pumped micro-cavity fiberlasers 60(1) . . . 60(N) that are formed or epitaxially fabricated onthe same substrate. Each laser 60 is substantially identical to laser 10shown in FIG. 1. However, lasers 60(1) . . . 60(N) may either be allintegrated on the same common substrate 16 as shown in previousembodiments, or may have a common substrate 62 as shown in thisembodiment, depending upon from which end of the laser array epitaxialgrowth is initiated on a substrate 16 or 62. Alternatively, lasers 12 inFIG. 3 may be separate or discrete lasers each having their ownsubstrate. In any case, in FIG. 3, lasers 60(1) . . . 60(n) areillustrated as being formed on a single substrate 62 and have a commonlower DBR mirror 64. Lasers 60(1) . . . 60(N) may all have the sameupper DBR mirror 17 or these mirrors 17 may be constructed as differentdielectric mirrors 17(1) . . . 17(N) such that each respective lasersmay operate at a different pump emission wavelength. Such an array mayalso be similar to the arrays illustrated in U.S. Pat. No. 6,693,934,which patent is incorporated herein by its reference, where theintegrated VCSEL elements are all fabricated to have different cavitylengths so that each individual VCSEL element emits a different emissionwavelength. The embodiment of FIG. 3 has particular utility for manyspectroscopy applications where additional wavelengths in the absorptionspectrum of an analyte under examination are required in order toextract an accurate measurement of the concentration of analyte in aspecimen, for example, at features comprising absorption peaks at aplurality of wavelength positions along a portion of the absorptionspectrum of the analyte, which peaks may number, for example, from 6 to10 or more comprising such absorption wavelength positions of interest.For these applications, multiple VCSEL pump laser/fiber laser units60(1) . . . 60(N) may be integrated to form a multiple emitter arrayhybrid device. An example of a preferred embodiment comprises the VCSELpump wavelength for lasers 12 to be at 785 nm so that the VCSEL pumplasers 12 may all be fabricated on a common substrate 62 and consist ofa linear or a two dimensional array, which would be butt coupled orotherwise optically coupled to a linear or two-dimensional array ofmicro-cavity, doped fiber lasers 12. In another preferred embodiment,the cavity lengths of the VCSEL lasers may be varied across the pumplaser array as taught in U.S. Pat. No. 6,693,934 so that each of theVCSEL pump lasers 12 in lasers 60(1) . . . 60(N) can provide differentemission wavelengths that are also designed to be at absorptionwavelength positions of interest of the analyte, i.e., spectralcharacteristic features across the characteristic optical spectrum ofthe analyte that are in the range of possible emission wavelengthsprovided by VCSEL pump lasers 12. When multiple VCSEL laser/fiber hybridlaser modules are incorporated as a group, modulation frequencies, suchas tone frequencies F₁, F₂ or F₃, or more, may be applied separately toeach pump laser 12, which is discussed in greater detail later on, andsuch modulation tone frequencies must be selected so as to avoid anyinterference between the modulation frequencies of different modules.Since the individual signals can be narrow band while the spectralbandwidth available is relatively large, the proper selection of tonefrequencies for multiple modulation of each pump laser 12 can easily beaccommodated.

The foregoing laser structures of FIGS. 1-3 are deployed in thisinvention to basically provide a plurality of different wavelengths foruse in analyte concentration determination in a given in vivo or invitro specimen. Such combination laser structures have not been knownthat I am aware of in the semiconductor pump laser/fiber laser art. Insuch a radiation source, the optical emission frequencies of the VCSELlaser 12 and the fiber laser 14 have greatly different electricalmodulation frequency dependent characteristics, as is known by thoseskilled in the art. It is well known that semiconductor lasers can bedirectly modulated by electrical injection at frequencies well in excessof 1 GHz with little difficulty. This is a result of the relativelyshort excited state carrier lifetime in a semiconductor laser, which isin the nanosecond range for spontaneous emission and decreases into thesub-nanosecond range for stimulated emission. In contrast, the excitedstate electronic lifetimes in a fiber laser are longer by several ordersof magnitude. This is indicated in FIG. 14 by the excited state lifetimevalues indicated adjacent to the states corresponding to lasertransition excited states in the thulium atom on the right side of thatfigure. These lifetimes typically range from a fraction of a millisecondto several milliseconds, corresponding to modulation responsefrequencies of one hundred or several hundreds of Hz range to perhapsinto tens of KHz range. Reference to modulation frequencies in theseranges and any intermediate ranges is referred herein as tonefrequencies to distinguish those lower frequencies from higher frequencymodulations as seen, for example, in the optical telecommunicationindustry in the GHz range. By simultaneously modulating the VCSEL at twotone frequencies, one greater than 10 KHz and one less than 100 Hz,modulation of its radiation will be imposed on the VCSEL 12 at both ofthese electrical frequencies, while modulation will be imposed on fiberlaser 14 only at the low modulation frequency, as the fiber laserresponse at the high modulation rate is extremely small and basicallynonexistent. Therefore, in the two wavelength emitter system such asillustrated in FIGS. 1 and 2, it is possible with a single suitablebroadband detector to measure the signal level at both of thesewavelengths, without employing a spectrometer, or employing wavelengthdispersive elements to differentiate between different wavelengthoutputs from the hybrid laser 10, 50 or 60. Thus, by analyzing thestrengths of the detector signals at higher and lower tone frequencies,the signal strengths at the two wavelengths can be deduced without theneed for a dispersive element, such as a diffraction grating or thelike, to optically separate the different wavelengths which is notnecessary or required in the context of this invention. Instead, thedifferent frequency signals to the same photodetector can bedistinguished in the electrical domain from one another by theirdifferent frequency rates as well as their frequency responses to twodifferent types of lasers 12 and 14 having highly different and separatemaximum frequency responses.

As a particular example, if fiber 34 has a core containing thulium whichis generally doped in the range of 2,000 ppm to 10,000 ppm and VCSEL 12is designed relative to formation of its respective cavity reflectors ormirrors 18 and 22 for a given cavity length providing an emissionwavelength of about 785 nm, fiber laser 14 will be pumped by thisemission wavelength providing fiber laser emission wavelengths at 1480nm, 1900 nm or 2,300 nm. The 2,300 nm wavelength range is of particularinterest because it is within a wavelength range that is difficult toreach with any conventional semiconductor lasers. This wavelength rangeis of particular importance in this disclosure since it containsspectral absorption intensities or other features, such as what isreferred to as spectral residuals, which are useful analyte signaturesin many important blood analytes, including glucose, while providing atthese same feature positions along the absorption spectrum relativelylow absorption for water. In this case, cavity reflectors (MF1) 36 and(MF2) 38 may be, respectively, 99.99% and 99% reflecting of 2,300 nmlaser radiation in the fiber mirror cavity, L3, but not reflective forfiber laser lasing at 1,900 nm, which is another possible lasingwavelength for thulium as seen from FIG. 14. Also, with a third mirror40 at the forward end 34A of the fiber cavity, L3, VCSEL 14 operates asa laser in two separate cavities, L1 and L2, where, for example, DBRreflector (R1) 22 may be 99.99% or highly reflective of the 780 nm pumpwavelength, DBR reflector (R2) 18 may be 99.10% or partially reflectiveof the 780 nm wavelength, and output reflector (R3) 40 may be 99.90% orhighly reflective of the 780 nm pump wavelength while only 1% or lessreflective of the fiber laser 2,300 nm radiation but permitting some ofthe pump radiation to pass with the laser emission output 39. Thus,output 39 of laser 10 comprises a combination of two separate peakoutput wavelengths of 780 nm radiation and 2,300 nm radiation.

Another example relative to a thulium doped core double clad fiberlaser, is to design reflectors (R1) 22 and (R2) 18 to provide a pumpingwavelength at 1,100 nm as can be seen from the energy-level transitiondiagram for thulium. As can be seen in FIG. 14, there are a set oftransition spacing of energy levels for thulium that the thulium atomcan absorb multiple 1,140 nm photons. In this case, cavity mirrors 18,22 and 40 for the pumping wavelength at 1,140 nm are designed to berespectively reflective at 99.90%, 99.10% and 99.99% of this pumpingwavelength whereas the fiber cavity reflectors 36 and 38 are designed tobe reflective of 480 nm radiation but 1% or less reflective of the pumpradiation of 1,140 nm. Thus, in this case, the hybrid laser emissionoutput 39 of laser 10 will be a combination of two separate peak outputwavelengths of 480 nm radiation and 1,140 nm radiation. Thus, it can beseen that the semiconductor laser pumped small- or micro-cavity fiberlasers 10, 50 and 60 can provide a plurality of different wavelengths inoutput 39 that, if designed properly, have utility in many differentapplications including optical spectroscopy which is the subject of thisdisclosure, as well as any other application that may require a compactlaser generator that provides multiple wavelength outputs and manypotentially selective wavelengths in different regions or areas of theinfrared spectrum. An example of another application for such a laserutilized in this invention is in LIDAR.

A structural example of the foregoing multi-wavelength application of asemiconductor pump laser/micro cavity fiber laser is illustrated in FIG.4. In FIG. 4, the semiconductor pump laser/micro cavity fiber laser 10Ais the same as laser 10 in FIG. 1 except that, now, the semiconductorpump VCSEL 12 is pumped with two different tone frequencies from tonegenerators 70 and 72 comprising a lower tone frequency, F₁, and a highertone frequency, F₂. These tone frequencies, F₁ and F₂, are chosen sothat F₁ is below the maximum frequency response of fiber laser 14 and F₂is above the maximum frequency response of fiber laser 14. In otherwords, fiber laser 14 will only respond to the modulation frequencybelow its maximum frequency response, in the case here the lower tonefrequency F₁, response. In this regard, see again FIG. 14 for the photonlifetime values at different excited states. This phenomenon of dopedfibers allows for determination of the laser energy content at twowavelengths by the examination of two different electrical frequencycontents: one fiber laser tone frequency appearing at output 39 whenmodulation is below the maximum frequency response at F₁ at an emissionwavelength from fiber laser 14 of laser 10A and two pump laser tonefrequencies, F₁ and F₂, appearing at output 39 regardless as to whetheror not the tone frequencies are above or below the maximum frequencyresponse of fiber laser 14. The net tone at F1 containing both fiberlaser and pump laser contributions, while the net tone at F2 willcontain only pump laser contribution. From the amplitudes of these twotones, the independent amplitudes of the energy at the two laserwavelengths associated with the fiber laser and the pump laser can beobtained. Thus, two useful emission wavelengths are achieved by allowingfor partial extraction of the pump radiation in conjunction with asingle emission wavelength from fiber laser 14, provide two differentand distinct modulated energy contents relative to two outputwavelengths derived by modulating the semiconductor pump laser 12 at twodifferent frequencies, one above and the other below the maximumfrequency response of fiber laser 14.

The following is a specific example of the foregoing relative to athulium doped fiber laser 14 for laser 10A. The maximum frequencyresponse for fiber laser 14 is approximately around or above 10 KHz.Thus, in order for fiber laser 14 to respond to a modulating tone, thetone must be at a frequency below this tone frequency, such as at afrequency around or below 100 Hz, which easily modulates the fiber laseroutput. Thus, for example, if frequency driver 70 is operating at F₁=100Hz and frequency driver 72 is operating at F₂=10 KHz for a pump laser 12that is designed to operate at an emission wavelength of about 780 nm,then for a thulium fiber laser 14 that has reflectors 36 and 38 set forany one of the emission wavelengths of 1,480 nm or 1,900 nm or 2,300 nm,the fiber laser emission output will be at one of these designedwavelengths modulated at a frequency, F₂=100 Hz, and the pump laseremission output will be at 780 nm modulated at both or two tonefrequencies, F₁=10 KHz and F₂=100 Hz. The emission output 39 is thenreceived by a broadband photodetector 74 with no intervening opticaldispersive elements, as previously explained. The resulting detectorsignal will contain frequency components at F₁ and F₂, enabling theindependent determination of the output power at 780 nm from theamplitude of the signal at F1, and subsequently determining theamplitude of the signal at 1,480 nm, 1,900 nm or 2,300 nm by subtractingthe amplitude of the signal at F₁ from the amplitude of the signal atF₂.

Light sources 10A or 50 modulated in accordance with the discussion ofFIG. 4 are well suited for application in two-wavelength spectroscopy,such as for blood oximetry, where the difference in absorption of blooddepending on its oxygenation state is employed to deduce the level ofoxygen in the blood, and the measurement is typically performed bycomparing the absorption at a pair of wavelengths. In this connection,reference is made to FIG. 6 where absorption spectrum of blood whereoxygenated hemoglobin is shown at curve 76 and deoxygenated hemoglobinis shown at curve 78. It can be seen from FIG. 6 that there aresignificant variations in the absorption of hemoglobin according to itsoxygenation state in the wavelength range from about 600 nm to 1,000 nmas indicated at range 79. In this range 79, there are significantcontrasts between oxygenated hemoglobin at curve 76 and deoxygenatedhemoglobin at curve 78. The difference in the spectra at points alongthese curves can be employed to measure the blood oxygen content. For ablood oximetry application, an erbium doped fiber for fiber laser 14 ineither laser 10A or 50 is particularly well suited, as it can also bepumped at 785 nm as well as made to emit at 980 nm or 650 nm. See thetransition diagram for erbium in FIG. 15 where these wavelengths areapproximately depicted in the transitions from excited states I_(11/2)and IF_(9/2), respectively. Either of these pairs of transitions(785/985 nm or 785/650 nm) may be employed in conjunction withtechniques known in the blood oximetry art to extract blood oxygenconcentration from relative absorption measurements. An alternativelaser 10A or 50 for this application, which may be simpler, is tomeasure the oxygen blood content by measuring the relative absorption at800 nm and 1,065 nm, wherein these two radiation wavelengths areacquired by employing a Nd doped fiber for fiber laser 14 with itsmirrors 36 and 38 set for emission wavelength at 1,065 nm. Such acompact oximetry device could be clipped to one's finger, such as duringsurgery, to measure simultaneously the blood oxygen content of thepatient as well as the patient's pulse.

Currently, blood oximetry is performed using a pair of light emittingdiodes, typically in a pulsed manner, and typically by analyzing thecomponent of the detected signal that is further modulated by the heartrate of the patient being monitored, such as, for example, duringsurgery. As an example, these LEDs may have major wavelength emission inthe range 79 shown in FIG. 6, such as, for example, at wavelengths 650nm and 980 nm, based upon spectral variation as seen in range 79 of FIG.6 relative to curves 76 and 78 at these wavelength vicinities. Thismethod results in a level of imprecision due to a variety of optical andbiological interferences. However, the above exemplified dual wavelengthsources 10A or 50 utilize more precise specified wavelengths, moreelaborate modulation techniques, higher overall radiation levels, and,optionally, more than two wavelengths, to provide greater precision inmeasurement determination with better SNR as well as better immunity tonoise and skin pigmentation variations.

With respect to the forgoing discussions relating to thulium transitionstates as shown in FIG. 14 and erbium transition states as shown in FIG.15, spectral monitoring at wavelengths outside of those available withthe employment of the emission bands of Thulium doped fibers may besupplemented with rare earth emission bands of other rare earth speciessuch as erbium. It should be noted that an erbium doped fiber has anabsorption band which overlaps well with the absorption band of athulium doped fiber in the 785 nm range, and the overlap is sufficientto provide for the pumping of both Er and Tm doped fibers bysemiconductor lasers fabricated on a common substrate. This potentiallyleads to great reduction in the complexity of optical spectroscopysystems which incorporate mixtures, for example, of Tm and Er for dopedfiber cores in fiber lasers 14. The benefit to the additionalavailability of Er is indicated in FIG. 7, where it can be seen that theemission wavelengths available to Er are complementary to those of Tm inboth regions identified as the short wavelength band and the longwavelength band, which are also identified as such in FIG. 8. Note thatthe emission wavelengths available cover wavelength ranges longer andshorter than the thulium wavelengths, as well as an emission band thatcovers one of the gaps in the thulium emission range. Thus, Er dopedfibers can provide additional capability and functionality in the laserspectroscopy technique of this invention. This advantage can be extendedby allowing for simultaneous doping of a single fiber co-doped both withEr and Tm, in which case cross-relaxation processes can be utilized topreferentially populate specific transition energy levels to provide forenhance emission at one desired wavelength relative to another undesiredwavelength.

While in FIG. 4, semiconductor small-cavity fiber laser 10A provides fortwo outputs based upon two different tone frequency modulations, thereare numerous examples of applications where more than two-wavelengthspectroscopy may be required. For some of these applications,three-wavelength spectroscopy may be sufficient. Further, for manyoptical spectroscopies where more than three wavelength emissions arerequired, it is of substantial benefit, for the sake of compactness andsimplicity, to extract as many useful, multiple tone modulated,multi-emission wavelengths from a given single multiwavelength lasercavity as is physically possible. FIG. 5 illustrates a semiconductorsmall-cavity fiber laser 10B which provides for three wavelengths inoutput 39 where pump laser 12 is modulated with three tone frequencies.Three useful emission wavelengths can be achieved by allowing forpartial extraction of the pump wavelength radiation at about 780 nm, forexample, in conjunction with two wavelength emission linewidths fromfiber laser 14. VCSEL 12 is modulated at three different electricalfrequencies, F₁, F₂ and F₃ via tone generators 80, 82 and 84, a firstabove and a second below the maximum frequency response of fiber laser14 and a third frequency is an intermediate frequency at which one fiberlaser transition responds but not another. This allows extraction of theenergy content at the two different fiber laser emission wavelengthsplus the pump wavelength by analysis of the three different modulatedfrequencies present in output 39 and are all detected by broadbandphotodetector 74 without the requirement of any intervening opticaldispersive elements, as previously discussed. Mirrors 36 and 38establishing the fiber cavity for fiber laser 14 will emitsimultaneously at two emission wavelengths. In this connection, observein FIG. 14 that when hybrid laser 10B is pumped at 780 nm via VCSEL 12,there are two parallel emission routes which both lead to the excitedstate of the 1,900 nm transition. These two parallel paths are the 2,300nm transition and the 1,480 nm transition. Since both transitions leadto the excited state of the 1900 nm transition, it is possible, withappropriate design of mirrors, to arrange for simultaneous emission offiber laser 14 at both 1,900 nm and either 2,300 nm or 14,80 nm. Thispair of emission linewidths, combined with the emission wavelength fromVCSEL pump laser 12, provides for a three-wavelength laser radiationsource 10B. This three-wavelength source may also be used for opticalspectroscopy without an additional spectrometer, by appropriate multiplemodulation of VCSEL pump source 12. As indicated in FIG. 14, thelifetime of the 2,300 nm/1,480 nm upper level state is 1.4 ms, while theupper level of the 1,900 nm transition has a lifetime of 6 ms. Thus, themodulation rate of the 1,900 nm emission is significantly lower thanthat of the 2,300 nm or 1,480 nm transition. As a result, it can be seenthat by selecting three different modulation tone frequency rates forVCSEL pump laser 12, there are three frequency signals, one to whichonly VCSEL 12 responds, one to which VCSEL 12 and fiber laser 14responds at the 2,300 nm/1,480 nm transitions, and one to which allthree transitions respond relative to both lasers. From the relativestrengths of these three different modulated radiation output signals atoutput 39, the output power levels at each of the three wavelengths canbe readily determined. Additional information on the relativeintensities of the wavelengths can be derived from the phase of themodulation, since as the modulation rate approaches and exceeds thecavity electronic lifetime for a given transition, the phase of theresponse begins to lag the phase of the electrical modulation, resultingin a phase shift in the detected signal which can also be translatedinto the intensity of the emission signal at a given wavelength.

A specific example of the three wavelength laser 10B is as follows. Pumplaser 12 has an emission wavelength of λ₁ of 780 nm. Fiber laser 14 hasits mirrors 36 and 38 designed to provide emission wavelengths λ₂ andλ₃, 1900 nm and 2300 nm (or, alternatively, 1480 nm). Three modulationtone frequencies are applied via frequency generators, generator 80 atF₁, generator 82 F₂ and generator 84 at F₃ where F₁ is, for example,greater than 10 KHz, F₂ is, for example, around 1 KHz and F₃ is, forexample, around 100 Hz. As a result, hybrid laser output 39 will containthe three signal wavelengths at λ₁, λ₂ and λ₃ where λ₁ will be modulatedat F₁, F₂ and F₃, λ₂ will be modulated at F₂ and F₃, and λ₃ will bemodulated at F₃. Photodetector 74 will detect F₁ which is proportionalto the optical power at λ₁, F₂ which is proportional to the opticalpower at λ₁ and λ₂, and F₃ which is proportional to the optical power atλ₁, λ₂ and λ₃. The detected amplitude at these frequencies, whentransmitted, reflected, or scattered from a sample being measured, is anindication or measurement of the spectral absorption of an analyte underexamination that has known peak features of peak intensities, forexample, at λ₁, λ₂ and λ₃, such as absorption positive peak intensitiesor negative peak intensities across a portion of the characteristicoptical spectrum, such as the absorption spectrum or spectral residualsof the analyte.

Therefore, a primary feature of this invention is a hybrid surfaceemitter or side emitter pump laser/fiber laser unit 10A or 10B or 50which simultaneously emits at a plurality of wavelengths, and for whichthe amplitudes of oscillation at the plural wavelengths can beindependently determined by modulation of the pump laser 12 or 52 at anappropriate set of frequencies, without the requirement of an opticalwavelength dispersive element, and with detection of the multipleamplitudes at the multiple frequencies determined by employing a singlephotodetector and a frequency analyzing circuit.

As previously indicated to some extent before, fiber laser 14 can bedoped with different kinds of rare earth species. In graphic diagram ofFIG. 7, there is shown a summary of the emission wavelength rangesaccessible to a key set of rare earth ions, including holmium,neodymium, thulium or erbium. The fiber emission wavelength ranges areillustrated with examples of different rare earth species as doped ineither silica fibers or ZBLAN fibers, which fibers are known in the art.Shown are the potential wavelength ranges for either fiber type that maybe employed as fiber lasers 14 with a core doped with holmium,neodymium, thulium or erbium. The wavelength ranges or bands areillustrated in the diagram by the grey-textured rectangles for each suchrare earth doped ZBLAN fiber or silica fiber. It can be seen clearlyfrom FIG. 7 that the spectral range of emissions for the different coredoped fibers is significantly affected by the host glass material of thefiber which could also include other glass materials such as, forexample, fluoride glass fibers.

Also shown in the diagram of FIG. 7 near the bottom is the emissionwavelength ranges for VCSEL lasers 12. For VCSEL lasers having pumpemission wavelengths near or around 800 nm, a set of emissionwavelengths for depicted fiber lasers span a broad bandwidth of thewavelength spectrum including those of interest for the examination ofanalytes, particularly those in blood. This means that all of thesewavelengths within the ranges depicted in FIG. 7 are available to thecombination of rare earth dopants and host glass fibers exemplified inthis diagram with pumping near 800 nm. As a further example, with theemployment of holmium, an energy transfer mechanism exists wherebyco-doping with thulium allows for optical pumping of thulium atoms inthe 800 nm wavelength band followed by energy transfer to the holmiumatoms results in fiber laser emission wavelengths also in the holmiumwavelength bands. Thus, all of the wavelength emission transitions shownin FIG. 7 are potentially available via a VCSEL laser pumping at oraround 800 nm.

Reference is now made to FIG. 8 which graphically illustrates theabsorption spectrum of water in the near infrared and peak positions ofspectral characteristic features of several biologically interestingspecies superimposed on the water absorption spectrum. Consideration ofthe water absorption spectrum is paramount because of the high watercontent in human biotissue and the strongly wavelength dependentabsorption of water which directly effects the optical penetration depthof impinging radiation into the biotissue specimen under examination. Asseen in FIG. 8, moving along the full length of the wavelength axis from800 nm to 3,000 nm, the optical penetration depth in water decreasesfrom nearly 1 meter to less than 1 micron. Also, note that the biotissuespectral characteristic features, such as peak intensities, for theanalytes, ethanol, albumin, urea, cholesterol and glucose in blood, havebroad absorption spectrums that overlap. Thus, in order to achieve ahighly accurate measurement of any one of these analytes in a biotissuespecimen, it is necessary to examine a plurality of these featuresacross the absorption spectrum at multiple wavelengths with highsensitivity in order to reliably extract a sufficient samplingcontaining relevant chemical concentration information from thespecimen. The accuracy as to examining wavelengths transmitted to theanalyte under examination, such as can be generated from the hybridlasers 10, 50 or 60, is an important aspect of this invention,particularly where the spectral characteristic features in the nearinfrared spectra of many of these aforementioned analytes are in closeproximity to one another. With hybrid lasers 10, 50 or 60, thepredetermined and necessary peak absorption wavelengths can be designedinto one or more hybrid lasers of this invention to provide for accuratemulti-wavelength spectral probing of any one of the species or analytesrelative to the others in same biotissue specimen.

As indicated previously, monitoring of blood glucose levels is a keyapplication of this invention. Glucose has important features in itsabsorption spectrum in the wavelength range from 800 nm to 3,000 nm,with particular significant features in the range of 2300 nm. Thesespectral features overlap well with the laser emission capabilities ofthe thulium doped fiber laser system. The absorption spectrum for otherkey analytes in the analysis of blood for glucose, some of which areshown in FIG. 8, are also prominent analyte features in blood, alongwith glucose and water. Greatest sensitivity to glucose will be obtainedby measurement at wavelengths where these other absorptions, other thanglucose, are lowest. In any case, data analysis is facilitated byacquisition of data at one or more of the spectral characteristicfeatures of glucose such as its absorption peaks as marked in FIG. 8 inthe short wavelength band and/or in the long wavelength band. Similarly,data on absorption at the peaks of other potentially interfering speciesor analytes, such as albumin, may be useful in reducing the number ofrequired spectral characteristic features required for glucose. Theseveral peak features in the glucose spectra are also wavelengths thatmay be added to the set of potential monitoring wavelengths. Thus, it isgenerally found that, by monitoring the spectral absorption orscattering at a number of wavelengths in the range, for example, fromabout 4 to about 16 spectral characteristic features, it is possible toextract, with good resolution, the glucose composition of a sample orspecimen.

With reference to FIG. 9, discussion now turns to further examination ofglucose absorption spectrum. Much is known about the opticaltransmission and scattering properties of glucose as set forth in amyriad of technical papers and patents in the field of glucose testingand analysis. Recent experiments utilizing FTIR spectroscopy within the2,000 nm to 2,500 nm spectral band has demonstrated important insightsinto the spectral structure of glucose-containing specimens. ProfessorMark Arnold and co-workers at the University of Iowa have performedresearch into noninvasive in vivo spectroscopy of blood analytes andhave achieved important results in the development ofstatistical/analytical models for quantitative analysis. Some of thiswork may be directed to the issue of wavelength number and featureposition of glucose across a portion of the infrared spectrum asillustrated in the article, J. T. Olesburg et al., “In VivoNear-Infrared Spectroscopy of Rat Skin Tissue with Varied Blood GlucoseLevels”, Proceedings of the SPIE—The International Society for OpticalEngineering, Vol. TBD, pp. TBD, 2004, which article is incorporatedherein by its reference. FIG. 9 herein is reproduced from that articleand shows glucose absorbance in μAU nM⁻¹ mm⁻¹ at dashed line curve 80for pure glucose as well as two superimpose analyte signal modelscomprising the net analyte signal at the solid line curve at 82 and thepartial least squares extracted glucose spectrum at the dash/dot curve84. These two model approaches illustrate what is called “spectralresiduals” illustrating a plurality of features comprising here bothpositive and negative peak intensities across the residual absorbancespectrum. The residual spectrum after removal of baseline factors fromthe glucose absorbance spectrum is simply the net analyte signal asdefine in the articles of Lorber entitled, Error Propagation and Figuresof Merit for Quantification by Solving Matrix Equations”, AnalyticalChemistry, Vol. 58, pp. 1167-1172, 1986 and of Lorber et al. entitled,“Net Analyte Signal Calculation in Multivariate Calibration”, AnalyticalChemistry, Vol. 69, pp. 1620-1626, 1997, which articles are incorporatedherein by their reference. It can be seen from FIG. 9 that curves 82 and84 they are highly comparable or almost trace one another indicating theaccuracy of spectral residuals for glucose in a glucose analyte sample.This provides, therefore, such as seen curve 82, a reliable net analytesignal (NAS) with spectral residual features, separate from or alongwith the absorption spectrum of pure glucose at curve 80, that can beutilized to determine glucose concentration accurately in the residualabsorbance spectrum, particularly over a spectral range of about 2,041nm to about 2,381 nm. As seen in FIG. 9, there are spectral residualfeatures along the NAS spectrum 82 at 2,047 nm, 2,071 nm, 2,128 nm,2,221 nm, 2,279 nm, 2,309 nm, 2,334 nm, 2,351 nm, and 2,361 nm. Notethat these spectral residuals provide significantly more spectralstructure than from pure glucose at 80. These multiple features providea sufficient spectral span and resolution that will be adequate forsampling the NAS at sufficient density to extract glucose concentrationsinformation. Thus, an optical spectroscopy system of this invention maybe restricted to a discrete set of wavelengths within this span in orderto determine the blood level of glucose, as demonstrated by the spectralfeatures of glucose at a number of positions as seen from FIG. 9. Asseen in conjunction with the fiber emission wavelengths of FIG. 7, manyof these spectral features or absorption wavelength positions, if notall, are achievable with the hybrid laser 10, 50 or 60 according to thisinvention through proper design of the laser cavity mirrors 36 and 38 aspreviously discussed. Although semiconductor lasers have beendemonstrated which can emit radiation in the 2,300 nm range ofwavelengths, none yet have proven to be reliable relative to efficiencyat modest power levels and reliable over time with spectral stabilitythat is required for this type of optical spectroscopy.

In the glucose monitoring system of U.S. Pat. No. 6,574,490, supra,there is illustrated in FIG. 1 of that patent a 45 watt tungsten-halogensystem having a sophisticated fiber optical system for radiationdelivery and collection, a single low noise NIR detector, and aspecially designed low noise Fourier transform infrared (FTIR) apparatusin order to achieve clinically meaningful noninvasive blood glucosedata. However, since the laser multiwavelength radiation sources of thisinvention are modulated with multiple tone frequencies relative todifferent generated wavelength signals, taking advantage of the muchslower response time of modulated fiber lasers relative to the muchfaster response of semiconductor lasers, two important improvements areachieved in system 90, as shown in FIG. 10, over the system as disclosedin this patent. First, the FTIR apparatus of patent '490 can becompletely eliminated, and the different optical probe wavelengthsgenerated by one or more hybrid lasers 60 can be readily identified bytheir modulation tone frequencies at photodetector 96. Second, theeffective photodetector bandwidth for each modulated wavelength signalcan be greatly reduced by the use of phase sensitive detectiontechniques resulting in far less laser power in the mW range required toachieve an acceptable SNR. The SNR of system 90 is superior over thesystem of patent '490 by 1.1 dB where that system requires a radiationsource of at least 3.87 W within the spectral range of interest from the45 W radiation source and a detector bandwidth of 1,000 Hz within theFTIR system in order to obtain the desired spectral resolution.

It is noted that a potential alternative to the halogen lamp of patent'490 as well as the multi-wavelength channel hybrid laser approach ofthis application is the replacement, in particular of either radiationsource with a light emitting diode (LED) or LED array. However, thespectral width of an LED emitting radiation in the 2,300 nm region isabout 200 nm with an output power of about 1 mW so that a group of twoor three LEDs would cover the combination spectral band covered by theoptical spectroscopy systems of the present invention. Although suchLEDs can be independently modulated in a manner similar to lasers, theirbroad spectral width greatly increases the sampled spectral range ofeach LED channel. Therefore, if the sampled point density requiredexceeds more than two or three data points or features over the 2,000 nmto 2,300 nm spectral range, it is most likely that LED source modulationwill provide inadequate resolution and also an optical dispersiveelement will be required as well, in order to separate out the differentwavelengths for examination. However, once such a dispersive element isinserted in the optical output, which is not required in the presentinvention, one loses the benefit of a compact system as well as much ofthe advantage of a phase sensitive detection system because opticaloutput will be greatly reduced. Also, LEDS have roughly three orders ofmagnitude less spectral power density than a 45 W halogen lamp. Thus,the LED spectral power density will be degraded by the same magnitude oforder, that is, three orders of magnitude or about 30 dB. Relative tothe FTIR/halogen lamp based system, in contrast, the system of thisinvention will equal or surpass the SNR of the FTIR/Halogen system.Thus, LED based systems are not a viable alternative to the system ofthis invention.

Reference is now made to FIG. 10 which illustrates in a block circuitdiagram a multiwavelength laser spectroscopy system 90 for use inconnection with the hybrid lasers 10, 50 or 60, previously discussed.System 90 comprises a multiwavelength laser subsystem 92 that may becomprised of hybrid laser 10, 10A, 10B, 50 or an array of hybrid lasers60 as illustrated in FIG. 3 as well as also seen in FIG. 11. Subsystem92 includes the laser modulated drivers as well as other controlelectronics, some of which is illustrated in the multiwavelength laserspectroscopy system embodiment of FIG. 12. As an example, subsystem 92may contain a set of five modulated lasers 60(1) . . . 60(5) with eachcapable of providing an output power of about 1 mW for a total subsystemof 5 mW of power. Subsystem 93 is the laser control subsystem thatcontrols the operation, including laser current biasing, as well as themulti-frequency modulation of semiconductor pump laser 12 or 52 whichalso is shown in more detail in FIGS. 12 and 13. The power of thesehybrid lasers is directed on a biotissue specimen at tissue analytesampling subsystem 94 where the output is detected by a broadbandphotodetector 96. Although the detector itself is broadband toaccommodate the total span of the several tones at which the severalpump lasers are modulated, the effective photodetector bandwidth formeasuring the amplitude of each of the tones may be reduced to very lowvalues, by the utilization of phase sensitive, or “lock-in” detection,as is well known in the art. Photodetector 96 may thus have an effectivedetection bandwidth of 0.1 Hz or less for each of the individual tonesto be detected as a measure of signal power. The modest power levels inthe 1 mW range are enabled by these narrow detection bandwidths. An evennarrower bandwidth than 0.1 Hz can be achieved for photodetector 96 byemploying digital lock-in amplifier techniques. A small portion of themulti-frequency output from subsystem 92 may be provided to lasercontrol system 93 as feedback via line 91 relative to control of signalintensity through bias change on pump lasers 12 or 52 as well aswavelength locking, although the wavelength outputs of these laser neednot be lock onto specific wavelengths, but the wavelength feedback maybe employed to insure that the emission wavelengths of laser 10 or 50are within a desired, predetermined bandwidth of operation. Subsystem 94includes a platform to either hold and position the example or may be ameans to properly hold the apparatus in the form of a handheld device ina predetermined relationship relative the in vivo tissues such as humanskin. As an example, subsystem 94 may have an engaging mechanism at thefront of the device to provide a predetermined gauging of themultifrequency output relative to the surface of the biotissue specimenunder examination such as an arm where photodetector 96 is adjacentlydisposed to collect scattered and/or reflected radiation for theoptically examined tissue. On the other hand, subsystem 94 can bedesigned to work in a transmissive mode where photodetector 96 is heldin opposed relation to the multi-frequency output with the biotissuespecimen positioned in the beam path between the output and thephotodetector, such as in the case, for example, in the employment ofhuman biotissue in the form of an ear lobe. Data collected byphotodetector 96 is provided in the form of photocurrent analog signalsupplied to data acquisition subsystem 98 wherein the lock-in amplifierwill typically be preceded by a transimpedance amplifier and minimaladditional analog processing such as low pass filtering to provide agood analog voltage signal which has a magnitude or amplitude componentand a phase component, prior to conversion to digital form via a highprecision analog to digital converter with about 24 bits of resolution.Once the signal is digitized, it is processed within a digital signalprocessor (DSP) which performs the function of multi-tone phasesensitive detection for the several tones simultaneously present on thephotodetector sensor. At the DSP, the amplitudes as well as phasedifferences of the multi-tone frequency signals from the biotissuespecimen are determined, which is data representing various wavelengthpositions or spectral features along the analyte wavelength spectrum orthe NAS spectrum, and is compared with known spectral data, stored inmemory, having experienced no spectral absorption which may,respectively, be employed to derive specimen modulated signal componentsand reference modulated signal components. It should be noted that theDSP also may typically provide for generation of the modulation tonefrequencies that are provided to the semiconductor lasers 12 atsubsystem 92. As between the reference modulated signal components andthe specimen modulated signal components, the difference betweenrespective modulation signals will be a DC value which is proportionalto the signal amplitude and the difference in amplitude between thereference signals and the specimen received signals which areproportional to the magnitude at a particular absorption wavelengthfeature of the analyte. This information is passed onto analyticalanalysis subsystem 99 for matrix and statistical analysis and finaldetermination from the multi-point or spectral residual data indicativeof the concentration of the analyte in the biotissue specimen underexamination, which is provided as an output 100. Furthermore, dataacquisition subsystem 98 also provides information to laser controlsubsystem 93 as to characteristics of the optical output relative tointensity which can also be correlated to an excessive variation inwavelength output, although small changes in wavelength are not criticalsince the bandwidth of sensitivity at the spectral features may have acomparatively broad linewidth.

FIG. 11 is a further embodiment of an optical spectroscopy controlsystem illustrated in connect with a plurality of array hybrid lasers60(1) . . . 60(N) where the multi-frequency/multiple wavelength outputsare provided to a specimen under examination, which is indicated at 75.In this embodiment, by comparison of the electrical modulation spectralamplitudes of the collected signal to those of the incident signal, thespectral absorption characteristics of an analyte, such as glucose, canbe deduced. From these absorption characteristics, and with appropriateselection of the set of wavelengths employed, the concentration of adesired analyte can be deduced. Multi-frequency/multiple wavelengthoutputs from array 60(1) . . . 60(N) are provided to beam splitter 72via a beam relay 71 where a small portion of the beam is extracted andprovided to reference detector/monitor 73 to analyze the output in termsof optical intensities as well as passing on to processor 79 the tonefrequency signals to be employed as reference frequencies in theemployment of a lock-in amplifier technique at processor 79. Dataanalysis and control processor 79 provides or otherwise synthesizes themodulation tone frequencies to be applied to each pump laser 12 in laserarray 60. These tone frequencies, as indicated previously, may be, forexample, a first tone frequency below about or around 100 Hz and asecond frequency above about or around 10 KHz; or a first frequencybelow about or around 100 Hz, a second frequency above about or around10 KHz and a third frequency intermediate of these two frequency rangessuch as around 1 KHz. There can be more than one such intermediatefrequency and these tone frequency examples can be varied relative themaximum frequency response of fiber laser 14. The first frequency isbelow and the second frequency is above the maximum frequency responseof any of the array fiber lasers 14. In the case of the third frequency,it is an intermediate frequency relative to the maximum frequencyresponse and active relative to at least one rare earth fiber transitionstate of a rare earth species in the fiber core but not at other suchtransition states of any other present rare earth species. However, itis within the scope of this invention that such an intermediatefrequency may activate more than one fiber rare earth transition state.

Returning now to FIG. 11, these modulation frequencies are provided tolaser drive and control circuit 70 where the different sinusoidalmodulation frequencies are provided to each laser 12 of hybrid lasers60. Lasers 12 may operate at the same pumping wavelengths or may eachoperate at different pumping wavelengths depending upon the positions ofthe spectral characteristic features of the analyte under examination.Also, the modulation frequencies provided by circuit 70 to each laser 12may be a plurality of the same modulation frequencies or may bedifferent modulation frequencies, in either case, following thecriteria, set forth above, i.e., one modulation frequency below and oneabove the maximum frequency response of the rare earth doped fiberlasers, and, in the case of a third or more frequencies, intermediate ofthe first two frequencies.

It should be understood that reference detector/monitor 73 may provideinformation to processor 79 as to the condition of the appliedmodulation frequencies in the output beam from hybrid laser array 60.Also, monitor 73 may also provide information about the intensity of thevarious wavelengths of radiation to laser drive and control circuit 70in order to correct those intensities to desired levels by means ofchanging the current bias to pump lasers 12 in array 60.

After passage from beam splitter 72, the multi-wavelength,multi-frequency beam is relayed to specimen under test at 75 by means ofbeam relay 74. In the schematic illustration here, beam relay 71 is mostgenerally a collimator of the diverging beam from the output of hybridlaser array 60 and beam relay 74 is most generally focusing optics tothe analyte specimen at 75. The radiation beam reflected from ortransmissive of the specimen is collected by beam relay 76 and providedto broadband laser detector/monitor 78 which may be comprised of aphotodetector which detects the modulated radiation and provides aphotocurrent replicating the multi-frequency modulations in the beamderived for the specimen under test or examination. The output ofdetector 78 is provided to data analysis and control processor 79 wherethe DC values of the various modulation frequencies are derived by alock-in amplifier technique where the respective tone frequenciesdetected prior to specimen 75 from monitor 73 are multiplied bycorresponding tone frequencies after specimen 75 from monitor 78providing a plurality of DC values representative of the amplitudedifferences between identical tone frequencies, before and afterspecimen examination. Additionally the tone frequency pairs may have aphase difference, each of which value is a further quantity useful forthe measurement of the amount of absorption by a specimen analyte at 75at various wavelength spectral characteristic features along theabsorption spectrum of the analyte.

A lock-in amplifier in processor 79 may also generate the plurality oftone frequencies employed for modulating each pump laser 12. The twosinusoidal frequencies of the reference signal and the specimen affectsignal are multiplied together to obtain a DC amplitude value. If thereis any difference in frequency between these two sinusoidal frequencies,the multiplied result will not be a DC value at zero frequency. However,multiplying the two frequency signals together which have the samefrequency and phase relationship, a DC signal is produced that is twicethe tone frequency. The signal is then passed through a low pass filteron the DC signal to obtain a narrow noise bandwidth and, therefore,improve the SNR to obtain a highly accurate amplitude value of thesignal at a given tone frequency. Thus, a multiplication is beingperformed of the same tone frequency at both at the input to and at theoutput from the absorbent analyte at 75 from which a DC signal isobtained that is proportional to the amplitude value at a givenabsorption spectral feature.

The matrix multiplication mentioned above is actually a two stepprocess. Photodetector 78 detects a set of N tone frequencies. Each ofthose N frequencies corresponds to N signals at N different wavelengths.So there is a first matrix multiplication to go from N tone frequenciesto N amplitudes at N wavelengths. Then, there is a second matrixmultiplication to go from N amplitudes at N wavelengths to determine theamount of concentration of the analyte in the specimen. The first matrixgoing from tone frequencies to wavelength intensities is reasonablydeterministic. It is knowing what the frequency response curves are aswell as knowing what the initially employed tone frequencies are. Thesecond matrix is more difficult to determine the elements in the matrixbecause it involves an analysis of statistics relative to multiplefactors starting with the effects that different human specimens willhave on the tone frequency signals. With the right matrix, you multiplythe amplitude of each of the frequency signals at different wavelengthsby a weighted value to obtain the analyte concentration value.

If the values of the matrix points for the different amplitude sampledpoints across the absorption spectrum under examination are known, theyare then multiplied by a predetermined weighted value determined by theabove mentioned statistics, for each point and a final weighted averagevalue of analyte concentration will be realized.

The relationship of the amplitude of the tone frequency signals at themultiple wavelengths both with and without the analyte absorbentcharacteristics affecting the signal provides a combination ofamplitudes that map into the concentration of the analyte in thespecimen under examination. This is a matrix multiplication wheredetermining the DC amplitudes of the frequencies are weighted ormultiplied by predetermined weighted values from which the amplitude orthe amount of the analyte in the specimen is determined. In other words,the derived digital signal is actually tapped and multiplied by aweighted value and the multiple values are added and averaged. Theseamplitudes are all different for different spectral positions ofwavelength spectral characteristic features in the absorption spectra ofthe analyte, depending also upon the amount of absorption at each givenwavelength.

The information electronically obtained at multiple wavelengthdistinguishing spectral features along a portion of the analyte opticalabsorption spectrum provides a reliable measurement of the analyteconcentration through multiple wavelength feature testing, comparisonand measurement. It should be noted also that data analysis and controlprocessor 79 is connected to laser drive and control circuit 70 toprovide other information relative to operating characteristics ofhybrid lasers 60 as will be explained in more detail relative to FIG.12.

In FIG. 12, additional detail is disclosed relative to themulti-wavelength laser subsystem 92 and the laser control subsystem 93of FIG. 10. N lasers 60(1) . . . 60(N) are formed on same substrate 62with an accompanying heatsink 28. Relative multi-wavelength lasersubsystem 92, temperature sensor, A, monitors the ambient temperature ofthe array substrate 62 via line 61 to telemetry chip 120. Suchmonitoring sensors are well known in the art and generally comprise athermistor. It is also within the scope of this embodiment thattemperature sensor may also include means to vary the temperature ofsubstrate 62 such as, for example, by way of a strip heater (not shown)secured between 62 and heat sink 28 with current supplied to the heatervia line 63 from telemetry chip 120. Also, each semiconductor laser 12of hybrid lasers 60 may have a voltage sensor from 1 to N to monitor thevoltage across the p-n junction of each pump laser 12. In addition, thelocal ambient temperature of each pump laser 12 may be monitored by alocal temperature sensor 1 _(A) to N_(A) which are connected totelemetry chip 120 via lines 122. Sensors 1A to NA measure thetemperature rise of each laser above the substrate temperature. Thetemperatures among lasers 12 vary because heat dissipation differs amongthese lasers. Further, the local ambient temperature of each rear earthfiber laser 14 of hybrid lasers 60(1) . . . 60(N) may be optionallymonitored with temperature sensors 1 _(B) to N_(B) which are connectedto telemetry chip 120 via lines 126. Thus, the electrical signals fromthese sensing devices are provided to telemetry chip 120 where lines 122are inputs to chip 120 of the voltage drop sensor values 1 to N acrosseach semiconductor laser 12, lines 124 are inputs of the local ambienttemperature values 1 _(A) to N_(A) for each semiconductor laser 12, andlines 126 are inputs of the local ambient temperature values 1 _(B) toN_(B) for each rare earth fiber laser 14. Telemetry chip 120 is adigital signal processor (DSP) chip that takes as input a plurality ofanalog signals, digitizes them via an analog to digital (A/D) converterproviding a series of data output at a given sampling rate that providevalues representative of the voltage operation and local temperature ofeach semiconductor laser 12 as well as the local temperature of eachfiber laser 14. This data is sent serially over a serial data link 119to control circuit 128 in laser control subsystem 93. Thus, this datarepresents three digital values for each laser 60 for N such lasers60(1) . . . 60(N) comprising the hybrid laser array. Since these analogsignals do not change rapidly over time, a less expense serial link,rather than a parallel link, is sufficient as the sampling rate of thesedigital values may be, for example, in the ms range or even up to asecond.

It is further within the scope of this disclosure that the operatingtemperature of fiber lasers 14 of hybrid lasers 60(1) . . . 60(N) may bevaried or adjusted by means of thermoelectric coolers or other suchPeltier type of coolers 127 as known in the art. Thermoelectric coolers(TECs) 127 operate by pumping heat toward or away from a surface of anelement required to be temperature regulated employing the Peltiereffect. This temperature control is of particular interest where theemission wavelengths of fiber lasers 14 are stable at a desired valuewhere it is important to maintain precisely these emission wavelengthwavelengths for particular analyte measurement applications. Thetemperature gradient in laser fibers 14 can affect the emissionwavelength of one or more fiber laser optical outputs 39(1) . . . 39(N).The temperature of fiber lasers 14 are monitored by temperature sensors1 _(B) to N_(B) and if any one of their temperature values significantlychanges to be offset from a desired or predetermined emission wavelengthof a given fiber laser 14, the desired operating temperature can bemaintained via chip 120 and input lines 125 to any one of coolers 127 sothat the corresponding fiber laser 14 operates precisely at desiredemission wavelength or with a desired linewidth. In addition, it iswithin the scope of this disclosure that heaters (not shown) also beincluded in combination with coolers 127 attached to each fiber laser 14to increase the dynamic range of temperature control of these lasers.

Alternatively, the information on the temperature of the individualfiber lasers may be used to deduce the emission wavelengths of thevarious lasers, and the algorithm for calculating species concentrationmay be adjusted to take into account thermal variations in the probingwavelengths. In this way, the additional cost of temperature controlsubsystems can be avoided.

In FIG. 12, further reference is now made to laser control subsystem 93which comprises control circuit 128 with accompanying lookup tables 129.Output from control circuit 128 includes control signals provided tolaser DC current bias 1 to N circuits 130(1) . . . 130(N) via a digitalto analog (A/D) converter circuit which is part of circuit 128. Thelaser drive currents provided by circuits 130(1) . . . 130(N) areprovide via laser modulation drive circuits 132(1) . . . 132(N) to pumplasers 12. Laser modulation drive circuits 132(1) . . . 132(N)perspective frequency modulation signals, F1 ₁, F1 ₂, F1 ₃, etc. to FN₁,FN₂, FN₃, etc., to pump lasers 12 along with the p-n junction currentbias. The foregoing mentioned real-time digital values for lasers 12 or14 from telemetry chip 120 are compared with values set for each suchlaser, such as derived during manufacturing testing, in lookup tables129 which tables are accessed via control circuit 128. Based upondifferences in values, the DC current bias of lasers 12 can be changedin order to change their output power level and also may be alsodeployed to increase the amplitude of the modulation current based uponreference information as to pump laser power levels and modulationamplitudes in lookup tables 129.

It may be asked why intensity monitoring of hybrid lasers 60(1) . . .60(N) is not accomplished by taking a portion of output 93 from eachlater via a photodetector to determined laser power levels. The approachof FIG. 12 replaces the more expensive photodetectors with relative veryinexpensive silicon IC circuitry and temperature sensors such asthermocouples, which can be fabricated in an integrated manner. Further,it is convenient in analyte testing not to interfere with the outputbeam path of hybrid lasers 60(1) . . . 60(N) and the intensity level ofmultiple lasers having multiple optical wavelength outputs, especiallyin testing situations where there is no ready access to the opticaloutput path of these laser outputs.

Reference is now made to FIG. 13 which is a modified version of thesubsystems 92 and 93 shown FIG. 12. In FIG. 13, it is to be noted that,rather than utilizing heatsink 28, a plurality of micro-thermoelectriccoolers (TECs) 136(1), 136(2), . . . 136(N) or other type of Peltiermicro-coolers are utilized to control the individual operatingtemperatures of each integrated semiconductor laser 12 for each hybridlaser 60(1), 60(2), . . . 60(N). Each of the coolers 136 is thermallyisolated from an adjacent cooler by means of a thermal isolator 138. Inthis embodiment, only the temperature of semiconductor pump lasers 12,as well as their junction voltages, are monitored via temperaturesensors 1 _(A), 2 _(A), . . . N_(A) and the monitored values arecompared with predetermined values in lookup tables 129 to determine ifa change is to be made to any one of these lasers relative to itscorresponding cooler 136 via lines 140 provided to each of the coolers136(1), 136(2), . . . 136(N) as shown in FIG. 13.

Reference is now made to FIG. 16 which primarily illustrates amulti-cavity laser 50A comprising a semiconductor laser for pumping amicro-cavity fiber laser which includes micro-cavity laser 14 buttcoupled to semiconductor side emitter laser 52. Thus, the description ofFIG. 2 is incorporated here by reference. Laser 50A thus is the same aslaser 50 except that the laser 50A shown here includes fiber Bragggratings 36FG and 40FG as cavity mirrors rather than deposited gratingsto form the multiple lasing cavities that exist in laser 50A. Likenumerical parts in FIG. 16 are the same as in FIG. 2 so that, asincorporated here, description of FIG. 2 equally applies to FIG. 16 forthese parts. As true in connection with FIG. 2, laser 50A has tworesonator cavities included in small cavity fiber laser 14, one cavityat the pump wavelength formed between grating 58A and front fiber Bragggrating 40FG at the out end of fiber laser 14 and a second cavity formedbetween front back fiber Bragg gratings 36FG. Fiber Bragg gratings 36FBand 40FB at the front or output end at beam 39 are set not to be fullyreflective of cavity light at the resonating wavelengths of the twolaser cavities so that these fiber gratings permit the extraction ofthem as output emission 39. To be noted is that micro-cavity fiber laser14 is butt coupled to edge emitter laser 52. An edge emitter laser 52 isfavored because it has potentially higher power relative to employing aVCSEL as a pump laser. However, it provides for reduced couplingefficiency with short cavity fiber laser 14 due to comparatively highercoupling losses, thus potentially favoring a VCSEL in applications withlower power requirements.

Fiber Bragg gratings or mirrors 36FG and 40FG are more favored thandeposited cavity mirrors as illustrated in the previous embodimentsbecause a greater degree of wavelength selectivity may be achieved. Itis now possible to form gratings directly into monolithic fibers withoutthe deployment of core dopants or hydrogen loading to bring aboutdesired refractive index changes employing an UV beam either associatedwith a phase mask or using UV laser interferometry. However, fiberswithout such grating enabling dopants or hydrogen loading may be used inthis disclosure, such as fluoride fibers with rear earth doped fluoridefiber cores incorporating, for example, Tm³⁺ in a ZBLAN fiber. Otherexamples are SiO₂ fibers with Ge doped cores additionally incorporatinga rare earth dopant such as Tm³⁺. However one may choose to utilize moreestablished UV methods in flouride fibers with the assistance of Ce³⁺ asan activation agent as disclosed in {Poignant, 1994#304} these gratingscan be preferred when compatibility with existing process lines andfabrication equipment is desired. See, for example, the article Poignant1994 No. 304 It is, therefore, not required, as indicated above, to havehydrogen-loaded fibers or fibers that are doped to permit refractiveindex changes employing a diffraction mask and a UV laser or laserinterferometry employing a UV laser. Instead, femtosecond pulsedinfrared source with a phase mask or femtosecond pulsed laserinterferometry is employed on such standard rare earth doped fibers toform the gratings, such as fiber gratings 36FG and 40FG in fiber laser14 of FIG. 16. See, for example, Androz et al., “Monolithicfluoride-Fiber Laser at 1480 nm Using Fiber Bragg Gratings”, OpticsLetters, Vol. 32(10), pp. 1302-1304, May 15, 2007; Wikszak et al.,“Erbium Fiber Laser Based on Intracore Femtosecond-Written Fiber BraggGrating”, Optics Letters, Vol. 31(16), pp. 2390-2392, Aug. 15, 2006;Mihailov et al., Bragg Gratings Written in All-SiO₂ and Ge-Doped CoreFibers With 800-nm Femtosecond Radiation and a Phase Mask”, Journal OfLightwave Technology, Vol. 22(1), pp. 94-100, January 2004; Smelser etal., “Generation of Pure Two-Beam Interference Grating Structures in anOptical Fiber With a Femtosecond infrared Source and a Phase Mask”,Optics Letters, Vol. 29(15), pp. 1730-1732, Aug. 1, 2004; Oi et al.,“Fabrication of Fiber Bragg Grating by Femtosecond LaserInterferometry”, Proceedings, pp. 776-777, 2001; Bernier et al., “BraggGratings Photo-Induced in ZBLAN Fibers by Femtosecond Pulses at 800 nm,Optics Letters, Vol. 32(5), pp. 454-4456, Mar. 1, 2007; and U.S. Pat.Nos. 5,946,085; 6,993,221; 7,031,571; and 7,257,294, all of whicharticles and patents are incorporated herein by their reference. Thesefemtosecond pulse-formed fiber grating processes may be applied equallyto subsequent embodiments to be discussed later.

FIG. 17 discloses multi-cavity hybrid laser 50B which is similar tolaser 50A of FIG. 16 except that the fiber gratings 36FG extend throughthe cavity of fiber laser 14 with a phase shift 150 provided between thetwo gratings. Thus, fiber gratings 36FG function as distributed feedback(DFB) fiber laser. This structure in FIG. 17 thus effectivelydistributes the reflectivity of fiber gratings G2 and G3 (36FG)throughout the length of the fiber laser cavity L3 and enhances theability of laser 50B to perform in a single longitudinal mode.

FIG. 18 is a more detailed energy-level transition diagram for thuliumillustrating efficiency improvement by the use of multiple wavelengthcascades. The cascade laser process involves increasing the rate atwhich electrons transition downwards from the pump transition excitedstate back down to the ground state by providing for stimulated emission(i.e. lasing) on as many as possible, if not all, of the energy leveltransitions that the electron must undergo. Further details of theoperation of cascade lasers may be found in the articles of Schneider,“Mid-Infrared Fluoride Fiber Lasers in Multiple Cascade Operation”, IEEEPhotonics Technology Letters, Vol. 7(4), pp. 354-356, April 1995, andPercival et al., “Highly Efficient CW Cascade Operation of 1.47 and 1.82mm Transitions in Tm-Doped Fluoride Fiber Laser”, Electronic Letters,Vol. 28(20), pp. 1866-1868, 24 Sep. 1992, both of which are incorporatedherein by their reference. Several different cascade laser schemesenable efficient photon extraction of cascaded photons when pumped witha pump source 52 in the range of 770 nm to 795 nm. Rear earth doped corefiber lasers of the cascade variety, as, for example, shown at first andsecond cascade in FIG. 18, can be realized with lasing at twowavelengths simultaneously such as illustrated in the first cascade orthe second cascade in FIG. 18. These cascaded lasers result in highefficiency due to rapid depopulation of the terminal laser level. Note,however, that such cascade operation necessitates a second pair ofgrating mirrors in order to establish a third resonator cavity, thethree resonator cavities comprising the composite pump resonator cavityL1 plus L2 and two fiber resonator cavities in L2 as seen in FIG. 19.

In FIG. 19, a pair of fiber Bragg gratings 152FG are include with thepair of fiber Bragg gratings 36FG, previously introduced in FIG. 16, toform the second fiber laser resonator between gratings 152FG operatingat an emission wavelength of the doped optical fiber 34. As an example,relative to the first cascade in FIG. 18, the 36FG formed cavity canoperate at an emission wavelengths of 1500 nm and the 152FG formedcavity can operate at 1800 nm. A first benefit realized here is that nowtwo fiber laser wavelengths become available in output emission 39. Asecond benefit realized is that laser power saturation effects andself-terminating effects can be eliminated by providing for fasterdepopulation of the terminal lasing level via stimulated emission whenthe terminal level has too long a lifetime. In such cases, electronswill tend to “pile up” in the terminal level creating a bottleneck whichwill reduce the lasing efficiency or, in extreme cases extinguish thelaser emission completely, a so-called self-terminating laser condition.The multiple cavity laser of FIG. 19, thus, has three resonator cavitiesthat include cavity L2, one at the pump wavelength and two at the pairof fiber laser emission wavelengths formed by fiber grating pairs 36FGand 152FG and the output fiber gratings 36FG, 40FG and 152FG permit theextraction of three wavelengths of emission at 39.

FIG. 20 is a further detailed energy-level transition diagram forthulium showing the multiphonon non-radiative combination at 155 in thesecond or 2300 nm cascade for thulium. Conventional 2300 nm cascadelaser devices rely on the third, approximately 3600 nm cascadenon-radiative multiphonon transitions to depopulate the ³H₅ level.Reliance on multiphonon relaxation from ³H₅ to ³F₄ results in “phononrelaxation bottleneck”, which can be relieved by providing a parallelpath for relaxation via photon-assisted stimulated relaxation, whichbecomes more rapid with higher photon density. This allows for higherphoton emission rates in the overall cascade system. Thus, thisdepopulation becomes a bottleneck at high laser power density. There isa benefit to constructing a third cascade 155 at 3600 nm point, in thesecond cascade that includes the 2300 nm cascade and the 1800 cascade,in order to provide a more rapid stimulated emission driven depopulationof the ³H₅ level than can be realized by multiphonon emission alone.This third level emission in the second cascade at the 3600 nmwavelength has many useful applications including optical spectroscopyapparatus for determining the concentration of analyte in a specimen asdisclosed herein.

FIG. 21 is an expanded energy diagram for thulium showing absorption andemission within the thulium atom when operated in the second cascade. Itis reported that there is rollover at high power in a 2300 nm cascadedlaser. The rollover results from the very high electron flux from the³H₅ to the 3F₄ levels as the total output power increases. This totalfux becomes greater than what can be accommodated for by the fixedmulti-photon relaxation rate, and thus becomes the rate limiting (andtherefore power limiting) step in the cascade process. See Caspary,.“Applied Rare Earth Spectroscopy for Fiber Laser Optimization” Berichteaus der Lasertechnik. Aachen: Shaker (2002)., incorporated herein by itsreference. The reason why is electron decay due to the fast buildup ofelectrons in the ³H₅ level but not fast enough. One signature of excesselectron buildup in the ³H₅ level is that electrons in this level mayundergo excited state absorption into the ¹G₄ level. From this levelthey may undergo spontaneous emission back down to the ground state,emitting a blue photon in the process. It has been recognized also thatthere is blue wavelength light emission as well as rollover. SeePercival et al, “Highly Efficient CW Cascade Operation of 1.47 and 1.82μm Transitions in Tm-Doped Fluoride Fiber Laser”, Electronic Letters,Vol. 28(20), pp. 1866-1868, Sep. 24, 1982, previously incorporatedherein by reference. The enhanced blue emission is due to excited stateabsorption (ESA) from the electron buildup in the ³H₅ level, as shown inFIG. 21, which competes with the desired cascade cycle. Reducinglifetime at the ³H₅ level can be accomplished by encouraging stimulatedemission at 3600 nm which will also improve the power available andefficiency at the 2300 nm and the 1800 nm emission wavelengths of thecascade. Thus, what results is a tri-resonant cavity fiber laser 50Dwhich is illustrated in FIG. 22.

The embodiment of FIG. 22 is the same as the FIG. 19 embodiment exceptcarries an additional pair of fiber Bragg gratings 154FG. As illustratedin FIG. 22, additional fiber Bragg gratings 154FG are incorporated intothe tri-resonator cavity fiber laser 14 to fabricate a third emissionwavelength in the 3600 nm cascade in the doped fiber 34. The other twopairs of fiber Bragg gratings 36F and 152FG are set to wavelengths inthe 2300 nm cascade and the 1800 nm cascade of the identified secondcascade. Thus, a trio of fiber laser wavelengths may be extracted at theemission output 39 along with a fourth wavelength at the pumpwavelength. An important benefit too is that power saturation effectsand self-terminating effects can be eliminated by providing for fasterdepopulation of multiple terminal lasing levels via stimulated emission.The overall result is higher output powers available for all of theparticipant wavelengths in the cascade. The four wavelengths, selectedby the grating mirrors for emission at emission output 39 can bedesigned to be at substantially matched to four distinguishing spectralcharacteristic features along at least a portion of an characteristicoptical spectrum of the analyte under examination.

While the invention has been described in conjunction with severalspecific embodiments, it is evident to those skilled in the art thatmany further alternatives, modifications and variations will be apparentin light of the foregoing description. While the approach of thisinvention has been exemplified mostly in regard to analytes involvingliving tissue, it will be understood by those skilled in the art thatthe principals of this invention herein may be utilized in examiningother specimens that may not include living tissue specimens wheremulti-frequency outputs of optical signals may be employed forstatistical examination, such as, for example, LIDAR, atmospheric gassensing, environmental monitoring and atomic/molecular spectroscopy ingeneral. Within this context, an analyte may be an atom or molecule inthe vapor phase, and a specimen may be a free space path between a laserand a detector containing some concentration of the vapor phase analyte.Thus, the invention described herein is intended to embrace all suchalternatives, modifications, applications and variations as may fallwithin the spirit and scope of the appended claims.

1. A method of determining the concentration of an analyte in a specimencomprising the steps of: providing a hybrid laser that comprises a firstand second laser for producing at a single output a plurality ofemission wavelengths that substantially match distinguishing wavelengthspectral characteristic features along at least a portion of acharacteristic optical spectrum of the analyte; modulating the firstlaser with a plurality of tone frequencies where at least one of saidtone frequencies is above a maximum frequency response of the secondlaser so that at least one emission wavelength from the second laser isnot modulated by a tone frequency; applying the modulated outputemission wavelengths to the specimen under examination; detectingradiation reflected, scattered or passed through the specimen; producinga set of values representative of wavelength spectral characteristicfeatures of the analyte within the characteristic optical spectrumportion from the applied modulated output emission wavelengths and thedetected reflected radiation; and determining the differences in theproduced set of values with predetermined reference valuesrepresentative of desired wavelength spectral characteristic features ofthe analyte to produce a final value representative of a measurement ofconcentration of the analyte in the specimen.
 2. The method of claim 1wherein the wavelength spectral characteristic features are amplitudeabsorption values at the absorption peaks along at least a portion of acharacteristic optical spectrum of the analyte.
 3. The method of claim 1wherein the step of correlating includes the steps of: multiplying eachof the set of values with a weighted value; and thereafter average theweighted values to produce a final value representative of a measurementof concentration of the analyte in the specimen.
 4. The method of claim1 comprising the further step of providing the first laser to be asemiconductor laser and the second laser to be a fiber laser.
 5. Themethod of claim 1 comprising the further step of modulating the hybridlaser with a plurality of tone frequencies.
 6. The method of claim 1comprising the step of providing the first laser to have an emissionwavelength to pump the second laser.
 7. The method of claim 1 comprisingthe further step of providing one of the emission wavelengths in thehybrid laser output to be an emission wavelength of the first laser. 8.The method of claim 1 comprising the further step of providing thesecond laser to have a maximum frequency response below that of thefirst laser; and applying tone frequencies to modulate the hybrid laserwhere at least one of the tone frequencies is non-responsive tomodulating at least one of the emission wavelengths from the secondlaser.
 9. The method of claim 1 comprising the further step of providingone of the emission wavelengths in the hybrid laser output to be anemission wavelength of the first laser.
 10. An optical spectroscopyapparatus for determining the concentration of analyte in a specimenthat utilizes a radiation source comprising at least one combination ofa semiconductor pump laser and small-cavity fiber laser where lasercavities of both lasers are butt coupled to provide a plurality of laserresonant cavities where the cavities are all, at least in part, formedby fiber Bragg gratings formed in the core of the laser fiber thatproduce a plurality of emission wavelengths at a same output of the buttcoupled laser that are designed to substantially match distinguishingspectral characteristic features along at least a portion of ancharacteristic optical spectrum of the analyte under determination. 11.The optical spectroscopy apparatus of claim 10 wherein the fiber lasercomprises a rare earth doped fiber wherein the fiber Bragg gratings areformed near or at the fiber ends employing a femtosecond pulsed infraredlaser process.
 12. A multi-cavity laser having a multi-wavelength outputat one end, comprising a semiconductor pump laser optically coupled to ashort cavity fiber laser having a rare earth doped core, themulti-cavity laser having a first resonator cavity formed by the lasercavities of the semiconductor laser and the fiber laser via a firstmirror at one end of the semiconductor laser cavity and a mirror at theone end and at least one second resonator cavity formed in the fiberlaser cavity with, respectively, a second and third mirror at each endof the fiber lasing cavity forming the second resonator cavity, thefirst, second and third mirrors formed as fiber Bragg gratings in therare earth doped core.
 13. The multi-cavity laser of claim 12 whereinthe doped fiber core of the short cavity fiber laser provides for atleast two cascaded transitions where the second and third mirrors waswell as fourth and fifth mirrors also at or near the ends of the fiberlaser cavity provide at the multi-wavelength output two wavelengthsgenerated from a two-step cascade transition.
 14. The multi-cavity laserof claim 13 further comprising a third wavelength is provided at theoutput at a wavelength of the semiconductor pump laser.
 15. Themulti-cavity laser of claim 13 further comprising a third wavelengthprovided at a non-radiative multiphonon transition in the two-step twocascaded transition.